Effects of brown seaweed extract, silicon, and selenium on fruit quality and yield of tomato under different substrates

Tomatoes (Lycopersicun esculentum L.) are an important group of vegetable crops that have high economical and nutritional value. The use of fertilizers and appropriate substrates is one of the important strategies that can assist in increasing the yield and quality of fruits. The present study aimed to investigate the effects of exogenous seaweed extract (Nizamuddinia zanardinii), silicon (Na2SiO3), and selenium (Na2SeO3) on quality attributes and fruit yield (FY) of tomato under palm peat + perlite and coco peat + perlite substrates. Seaweed extract significantly improved several of the fruit quality attributes such as total carbohydrate content, total soluble solids (TSS), and pH as well as the FY. The results showed that silicon (Si) (75 mg) was the best foliar spray treatment to enhance the fruit firmness (30.46 N), fruit volume (196.8 cm3), and FY (3320.5 g). The highest amount of plant yield (3429.33 g) was obtained by the interaction effects of silicon (75 mg L-1) under the effect of palm peat. The use of selenium (Se) led to improvements in flavor index (TSS/TA). Also, the application of palm peat + perlite substrate caused an increase in vitamin C (16.62 mg/100g FW), compared to other substrates (14.27 mg/100g FW). The present study suggested that foliar spray with seaweed extract and Si had beneficial effects on the quality and FY of tomatoes. Also, the palm peat substrate can be used as a good alternative to the coco peat substrate in the hydroponic system.


Introduction
Tomato (Lycopersicun esculentum L.) is one of the important vegetable crops and a popular product in many parts of the world. Tomato, as a major source of lycopene, is rich in potassium and vitamins and is an important component in the human diet [1]. To increase the quality of different fruit, so far, there have been strategies for the application of non-essential elements and fertilizers that contain amino acids, the increase or decrease in some of the macro-elements, accurate control of irradiance, and monitoring temperature conditions, as a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 According to previous reports, the performance of palm-date wastes as substrate was similar to, or even better than, peat, perlite, and coco peat substrates [21,22].
Considering the advantages of using silicon (Na 2 SiO 3 ), selenium (Na 2 SeO 3 ), and seaweed extract that help increase the performance and quality of various products, we monitored tomato plants as they grew in two different substrates after foliar spraying them with silicon, selenium, and seaweed extract under controlled conditions. The objectives of this study were to investigate: (i) the effects of foliar spraying silicon, selenium, and seaweed extract at different concentrations on the yield and quality of tomato; (ii) the effects of two different combinations of substrates (i.e. palm peat + perlite and coco peat + perlite substrates) on the yield and quality of tomatoes.

Plant material and treatments
The seeds of tomatoes were sown in polyvinyl chloride tubes (6 cm in diameter and 10 cm in length) in March 2020. After a month, the seedlings (4-leaf stage) were transferred to 8-liter pots. The experiment was a factorial type and was based on a completely randomized design with five replications. The factors included substrates (i.e. coco peat + perlite and waste of date palm + perlite) as well as seven treatments of foliar spraying, i.e. the control (normal conditions), two levels of Na 2 SiO 3 (25 and 75 mg L -1 ), two levels of Na 2 SeO 3 (4 and 10 mg L -1 ) and two levels of seaweed extract (10 and 20%) with five replications. The pots were filled with coco peat (50%) + perlite (50%) and waste of date palm (50%) + perlite (50%). One tomato plant was cultivated per pot. The plants were maintained in a greenhouse with natural light conditions (13-h photoperiod, with a relative humidity of 28 to 62%, and a photosynthetic photon flux density of 500 μmol.m -2 .s -1 at the canopy level). Plants were fertigated three times a day (200 mL) with Hoagland's nutrient solution [23] which was delivered at 06:00, 12:00, and 19:00 h. The foliar spraying was carried out with seaweed extract, Na 2 SeO 3, and Na 2 SiO 3 , which began to be applied 21 days after transplanting the seedlings and, subsequently, every one week up to 30 days after the transplanting.

Seaweed extracts
Algae Nizamuddinia zanardinii were collected in the Gulf of Oman off the coast of Chabahar (25˚20' N, 60˚27' E), Iran. The algae were washed with distilled water and dried for 8 days at room temperature and subsequently ground in a mortar. Then, the aqueous extract was produced by boiling 50 gr of algae powder in 500 mL of distilled water, and the extracts were filtered through a filter paper (Whatman No 1; Whatman, Pittsburgh, PA, USA) before storage [24]. The chemical characteristics of the seaweed are presented in Table 1.

Total carbohydrate content
Total carbohydrate contents in tomato fruits were estimated using the Anthrone reagent method [25] with slight modifications. Mature fruits were collected from plants and dried. About 100 mg of dried samples were weighed, followed by adding 10 ml of water into a test tube. The solution was kept in a boiling water bath (90˚C) for 50 min. One mL (1.00 mL) of the extract was taken from the test tubes and 4 mL of the Antheron reagent (3%) (200 mg/100 ml of ice-cold 95% sulphuric acid) was added to the extract. The mixtures were kept in a water bath (60˚C) for 30 min at 100˚C. The samples (2 ml) were taken in cuvettes and the maximum value of absorption was measured at 630 nm using glucose as a standard.

Total soluble solids (TSS), titratable acidity (TA), flavour index, and pH
The juice of flesh tissues (10 gr) was extracted from 8 fruits, using a domestic juice extractor, and then centrifuged at 12,000× rpm for 5 min. The supernatants were dropped onto a handheld digital refractometer. The content of TSS was expressed as % [26].
Total titratable acidity was estimated as described by Khan et al. [26]. Tomato juice was titrated with 0.1 M NaOH to an end point of pH 8.2. TA was expressed as percentages, based on malic acid. The ratio of TSS/TA was calculated as the flavour index [27]. Fruit juice pH was measured with a pH meter (HI-2211; Hanna Instruments, Padova, Italy).

Fruit firmness
The fruit firmness was measured at five spots on each fruit. Four fruits were randomly applied per replicate (5 replications per sampling time) using a handheld penetrometer (FT-327; UC Fruit Firmness Tester, Milano, Italy).

Vitamin C
Vitamin C content was measured as described by Contreras-Calderón et al. [28]. Tomato fruit juices (5 g) were mixed with 15 ml of 2 g/100 g oxalic acid solution and centrifuged at 12,000 rpm for 15 min. Then, 10 mL of the supernatant were titrated to a permanent pink color by 0.1 g/100 g 2, 6-dichlorophenolindophenol titration. Vitamin C content was estimated according to the titration volume of 2,6-dichlorophenolindophenol and expressed as milligrams per 100 g of fresh weight.

Yield
All fruits per plant from each treatment were harvested from the first to the fifth tomato cluster and their fresh weights and fruit volume were determined.

Statistical analysis
In this research, a factorial experiment was conducted based on a completely randomized design with five replications. Analysis of variance (ANOVA) was laid out, and means were compared using the Duncan Multiple Range test by the least significant difference (P = 0.01). The SAS 9.1 software (SAS Institute, Inc., Cary, NC) was used for data analysis.

Fruit firmness
Our results showed that the application of Si, Se, and seaweed extract significantly (P < 0.01) increased the rate of fruit firmness in the tomatoes. The highest value of fruit firmness (30.5 N) was obtained by the interaction effects of Si 75 mg L -1 under the effect of palm peat and the lowest value of fruit firmness (22.73 N) was observed in the control treatment (Table 3). Both Si treatments increased fruit firmness and the highest value was obtained in tomato plants that had been exposed to 75 mg L -1 Si (Fig 1). In addition, Se and seaweed extract also improved the firmness of the fruit compared to the control treatment. The application of seaweed extract had more beneficial effects on fruit firmness in comparison with the effect of the Se treatment (Table 3).

Total carbohydrate content
The total carbohydrate content is one of the important traits for the investigation of fruit quality. There was a significant difference among different foliar spray treatments for this measurable feature. Plants that had been treated with seaweed extract had higher (85.49 and 80.71 mg) mean values of this trait, compared with other treatments (Fig 2). As indicated in Fig 2, exogenous 4 mg L -1 Se application significantly decreased total carbohydrate content in fruits, which made it significantly lower (61.45 mg/100g) than the control treatment 66.85 mg/100g). Also, 75 mg L -1 Si application had useful impacts (75.61 mg/100g) on the total carbohydrate content. According to the results of ANOVA, there were no significant effects of substrate on total carbohydrate content ( Table 2).

Total soluble solids (TSS)
According to the results of the present study, different foliar spray treatments had a significant positive effect on TSS content in comparison with the control (Fig 3). TSS contents were significantly affected by the exposure of plants to different foliar spray treatments. Higher TSS contents were observed in plants that had been exposed to different concentrations of seaweed extract (6.97 and 6.48˚Brix) in comparison with the effect of other treatments under both substrates. In this study, the interaction of coco peat substrate and seaweed extract (20%) led to the highest content of TSS, although it had no significant difference with the interactions of date-palm peat substrate and the two levels of seaweed extracts (10 and 20%; Fig 4).

Total titratable acidity and pH
The analysis of variance revealed that the substrate treatments had no significant impact on the amount of titratable acidity (TTA) and pH ( Table 2). The amount of TTA and pH of tomato juice were significantly affected by the seaweed extract, Si, and Se, compared with the control, although there was no significant difference among these treatments (Fig 5). The higher mean values in both characteristics were observed in plants that had been exposed to Se (4 mg L -1 ) and seaweed extract (10%), respectively (Fig 5).

Flavour index and Vitamin C
Our results showed that foliar spraying tomatoes with seaweed extract (20%) and Se (10 mg L -1 ) caused higher levels of flavor index than plants foliar sprayed with Si or water (control) (Fig 6).  We observed that the application of seaweed extract (20%) led to increases of flavour index from 8.7 to 10. The results revealed that coco peat + perlite substrate had beneficial effects on the flavour index in comparison with palm peat + perlite substrate (Fig 6). As indicated in Table 2, foliar spraying with seaweed extract, Se, or Si, caused no significant difference in the amount of vitamin C. Our results showed that the amount of vitamin C was higher in tomatoes cultivated in the date-palm peat + perlite substrate (17 mg/100g FW) in comparison with coco peat + perlite substrate (14 mg/100g FW; Fig 7).

Fruit volume and yield
According to the results, the higher concentrations of seaweed extract and Si increased the FY and fruit volume more, compared to their lower concentrations (Fig 8). The highest value for the plant yield (3429.33 g) was obtained by the interaction effects of Si 75 mg L -1 under the effect of palm peat (Table 3). Also, based on the results, all treatments enhanced the FY (Fig 8).
However, Si (75 mg L -1 ) and seaweed extract (20%) increased the FY by 70 and 68%, respectively, compared with the control (Fig 8). Our results showed that the application of different substrates caused no significant difference in the FY and fruit volume ( Table 2).

Fruit firmness
Fruit firmness is one of the main factors in determining the quality of tomato fruits. It is extremely important in overall product acceptance [29]. Extended firmness is favorable for the long-term storage of tomato fruits by consumers. Huang et al. [30] showed that cell size, cell wall structure, turgidity, and membrane properties affected the firmness parameters. Our results showed that foliar spraying with Si substantially enhanced fruit firmness in tomato plants. Similarly, Si led to an increase in the fruit firmness among different plants such as apple, strawberry, avocado, and cherry tomato [31]. It seems that the application of Si probably decreased the rate of tomato softening by slowing down the process of cell-wall degradation while increasing stability in the cell wall structure by causing the binding of Si to the cell wall of pectin molecules [32]. On the other hand, it was reported that the deposition of silicon led to changes in the cell wall and increased the number of non-diffusible anion sites, which

PLOS ONE
adsorb Ca, thereby causing more absorption of Ca by increasing the fruit firmness [33]. Also, we observed that the application of Se on tomato plants improved fruit firmness in comparison with the control. Hernández-Hernández et al. [34] revealed that the application of Se could be a successful strategy for increasing the shelf life of tomato fruits by enhancing fruit firmness. The authors showed that enhancing firmness in tomato fruits, caused by Se, could have resulted from enhancements in the activity of the PAL enzyme and the lignification of the pericarp cell wall. Our results revealed that firmness was positively affected by seaweed extract.

TCC and TSS
The total carbohydrate and TSS contents, as indicators of fruit quality, were recommended by previous studies. The current data showed that foliar spraying with seaweed extract substantially affected the accumulation of TCC and TSS contents in tomato plants. We showed that the increase in seaweed extract concentration positively led to enhancements in the https://doi.org/10.1371/journal.pone.0277923.g008 accumulation of TCC and TSS in tomato plants. Similar to our results, it was reported that the application of seaweed extract (Sargassum johnstonii or Ascophyllum nodosum) led to accumulations of carbohydrates in various plants such as tomatoes [35]. This increase in the TCC of the plants, which had been treated with seaweed extract, may be attributed to the increase in vegetative growth characteristics [36]. Also, the increase in TCC might be attributed to the higher availability of macro and micro elements (K, N, Mg, Ca, Zn, and Ni) and better growth of roots by foliar spraying the seaweed extract [37]. Rasouli et al. [38] reported that the application of seaweed extract increased the photosynthesis rate and chlorophyll index in lettuce plants and, subsequently, caused the accumulation of carbohydrates. On the other hand, our results showed that the application of Si significantly increased the amount of TCC than the control. It was reported that Si had beneficial effects on the photosynthesis of tomato seedlings and the regulation of carbohydrate metabolism in cucumber plants [14]. TSS status of the tomatoes was largely influenced by exposure of plants to different foliar spraying treatments. These data were consistent with previous studies in which seaweed extract (Ascophyllum nodosum and Ecklonia maxima) enhanced the TSS in Allium cepa, Cucurbita pepo, and Vitis vinifera [39]. Also, Kumari et al. [35] reported that a higher concentration of total soluble sugars in tomatoes was observed in foliar applications of seaweed extract which confirms our results on the useful effect of seaweed extract on TSS content. The TSS content depended on the transported organic and ions solutes which are converted into sugar inside the fruit [40], which is where foliar spraying the seaweed extract increases sugar biosynthesis and contributes to a higher TSS content [41]. The application of Se led to an increase in TSS (20%) in comparison with the control treatment. Similar results were also obtained in a case where Se was added as a nutritional solution [42]. It was reported that the use of Se led to enhancements of starch accumulation and positively affected soluble solid accumulation in ripe fruits [43]. Our results suggested that the application of seaweed extracts and Se may positively enhance TSS, thereby positively affecting consumer preferences. In the same line, Mohammadi Ghehsareh et al. [44] showed that the TSS content of tomatoes cultivated in perlite or date palm substrates had no significant differences. These results showed that the date palm peat can be used as a substrate for hydroponic systems with similar effects to coco peat substrate. Also, as it has suitable physical properties, low cost, and high availability [45], therefore, this substrate can be a good alternative to coco peat in hydroponic systems.

TTA, pH, and flavour index
The TTA is one of the most important quality attributes, which determines the storage life, the stability of ascorbic acid, and the flavour of tomato products [46]. Our study revealed that the application of different substrates had no significant effect on TTA and pH (Table 3). These results were similar to previous studies by Mohammadi Ghehsareh et al. [44] on tomatoes grown in date-palm and perlite substrates. We found that the usage of seaweed extract, Si, or Se, significantly increased TTA contents and pH in tomato fruits in comparison with the control treatment, whereas the highest TTA contents and pH belonged to plants treated with the seaweed extract (10%). This study confirmed earlier reports that the application of seaweed extract, Si, and Se led to enhancements in the TTA and pH in different plants such as strawberries [36] and mango [47]. It was demonstrated that the usage of seaweed extract stimulated the biosynthesis of organic acids in a differentiated way and decelerated the natural process of senescence of different fruits, since the decline in fruit acidity was almost associated with the utilization of acids in the respiratory process, due to the process of maturation and fruit ripening [47]. Consistent with the current results, Karagiannis et al. [48] showed the increase of titratable acidity after Si foliar application due to the influence of Si on the decrease of ethylene production and respiration. These results indicated that the high titratable acidity content in tomato fruits by the foliar application of seaweed extract, Si, or Se led to a decrease in the need for adding acidifiers to fruit juice [49].
The ratio of TSS to TA is ordinarily applied as a flavour index for tomato fruit. The flavour index is called the maturity index. Low amounts of flavour index show correlated with acid flavour while higher amounts of flavour index caused a mild flavour due to the excellent combination between acidity and sugar content. These results are coherent with Peripolli et al. [50] report. Mohammadi Ghehsareh et al. [44] showed that higher amounts of flavour index were obtained by using coco peat + perlite as substrate in comparison with palm peat + perlite, perlite, and palm peat substrates. The application of coco peat as substrate in hydroponic systems probably provided a suitable environment for root growth and development, while acting against drought stress and restricting water consumption, thereby improving the flavour index.

Vitamin C
Vitamin C, as an antioxidant, is one of the major nutritional quality indicators of fruits [51]. It was demonstrated that the amounts of vitamin C in plants cultivated on various substrates (i.e. coconut fiber, peat moss, perlite, and bark) varied considerably (14 to 32 mg/100g FW) [52]. Previous results confirmed further our results on the useful effect of palm peat substrate on the increase in vitamin C content in comparison with the effect of coco peat. Ahmad et al. [44] reported that the available potassium in palm peat was instantly higher than that in coco peat substrates. It was stated that an adequate amount of potassium is essentially required to improve ascorbic acid which, thus, increases the quality of tomato fruits [53].

Fruit volume and yield
Our results showed that the highest amount of fruit yield (FY) was observed in palm peat substrate in comparison with coco peat, but these results had no significant difference (Table 3). Similarly, Mohammadi Ghehsareh et al. [44] reported that the FY of greenhouse tomato was not affected by the usage of cocopeat, perlite, and palm peat substrates. The present study showed that fruit volume and FY of tomato plants were instantly influenced by foliar spraying with seaweed extract and Si. Various studies revealed that the usage of seaweed extract as a foliar spray or soil application increased the yield components in different crops [41,54]. Seaweed extract included plant hormones (auxins, gibberellins, and cytokinins), trace elements (Zn, Cu, Fe, Mo, Ni, and Mn), amino acids, and vitamins [41]. It seems that the increase in fruit volume and FY by seaweed extract may be due to a higher increase in sink strength of the tomato fruits to attract more assimilates and water. Jarosz and Dzida [55] reported that the application of Si led to improvements in flowering and fruit set in tomatoes. In the present study, it seems that the enhancement of FY through foliar spraying with Si was explained by better photosynthesis, flowering, and fruit set [14]. Our results showed that the application of Se (4 and 10 mg L -1 ) significantly increased FY and FV in comparison with the control. The effects of Se on plants depended on the sensitivity of each species, the applied concentration, and the form of application [42]. It was reported that the application of Se in low dosages improved yield, while high doses of Se increased free radical production, causing oxidative stress [56]. It seems that the increase of resistance to oxidative stress can probably have key effects on the increase in tomato yield by foliar spraying with Se [57].

Conclusion
This research demonstrated that foliar application of seaweed extract, silicon, and selenium significantly enhanced the quality and quantity of tomatoes. In general, applications of seaweed extract (20%) and Si (75 mg/L) was more effective in improving total carbohydrate content, fruit firmness, TTA, pH, TSS, flavour index, and FY, compared with the rest of the treatments. In contrast to the foliar spray treatments, our results showed that the vitamin C content in fruits increased by the effect of date-palm peat substrate. The present study suggested that foliar spraying with seaweed extract and Si could enhance the quality and FY of tomatoes. Also, our results led to the recommendation that the application of palm peat can be a good alternative to coco peat in hydroponic systems.